1. Field of the Invention
The present invention relates to an apparatus and method for transferring frames, and more particularly to an apparatus and method for transferring frames using a logical link formed by aggregating a plurality of physical links.
2. Description of the Related Art
Most data communication networks transport data in the form of packets or frames with finite lengths. For example, transmit data is divided into frames at the data link layer before it is placed on a physical transmission medium. Those frames are delivered to their destination via intermediate network devices such as layer-2 switches. In this connection, link aggregation techniques are used to enhance the performance and quality of communication channels between network devices. Link aggregation, known as IEEE standard 802.3ad, refers to a technique to provide a single logical link by bundling two or more physical links (e.g., network cables) between two network devices.
Aggregated links provide an increased data bandwidth and thus make high-speed communication possible without using costly cables or expensive network interface cards. In recent years, many communication carriers have introduced link aggregation into their networks, but for the purpose of improving availability with link redundancy. By virtue of its simultaneous use of multiple physical links, the link aggregation prevents the communication channel from being completely disrupted even if some physical links fail.
Suppose, for example, that some physical links are selected as working links to constitute a logical link, while the remaining links are virtually blocked for the purpose of backup. The working links play the role of operating links for communication. When some of those operating links are disrupted, the system detaches the failed links from the link aggregation group and removes the virtual blocking from the backup links, thereby attaching those backup links to the link aggregation group (see, for example, Japanese Patent Application Publication No. 2004-349764). This technique uses two groups of physical links, one as working links to build a logical link and the other as backup links. In the event of a failure in the working links, the backup links are changed from unused state to usable state, while the working links are set to unused state. The link aggregation group thus continues to provide network connection.
Since a network failure leads to degradation of communication quality and system availability, it is desirable to minimize the downtime due to such failure. This is true not only in ordinary networks, but also in a network using link aggregation. Particularly, in the case of a carrier network employing link aggregation for redundancy purposes, instant switchover from failed links to normal links is required for quick recovery of communication.